United States president Barack Obama has blamed the government shutdown on an "ideological crusade" by Republicans to derail his healthcare program.

More than 800,000 government workers will spend a second day on unpaid leave after the shutdown came into effect at midnight on Monday (local time), when Congress failed to meet a deadline to bring in a new budget.

Despite this, the centrepiece of Obamacare - subsidised health insurance - is now being sold online across all US states, in a move Mr Obama says will allow tens of millions of uninsured Americans to get medical coverage.

Mr Obama says it is strange that Republicans have stopped the government without achieving their goal of stopping his reforms.

"DC cess-pols shut down government. They get paid while nation suffers," it trumpeted.

A few hours into the shutdown, Republicans in the House appointed delegates, or conferees, to negotiate with the Senate later on a spending plan to get the government up and running again.

But if they still want to tinker with Obamacare, the Senate will not negotiate, an aide to Senate Majority Leader Harry Reid said.

Mr Obama warned that a government shutdown could badly damage an economy which has endured a sluggish recovery from the worst recession in decades.

"We may not know the full impact of this Republican shutdown for some time," he said.

"But we do know a couple of things. We know that the last time Republicans shut down the government in 1996, it hurt our economy."

The president also warned Republicans against using a crucial mid-October deadline to raise the government's $US16.7 trillion debt ceiling as leverage to try to reverse the health care law or achieve other political objectives.

"Congress, generally, has to stop governing by crisis," he said.

"I'm not going allow anybody to drag the good name of the United States of America just to refight a settled election or extract ideological demands."

A debt default that would result if Congress fails to raise the debt ceiling when it is reached in less than three weeks could be devastating, Mr Obama said.

The threat of default in 2011 resulted in a painful debt rating downgrade, he added.

"If they go through with it this time, and force the United States to default for the first time in its history, it would be far more dangerous than a government shutdown, as bad as a shutdown is. It would be an economic shutdown," he said.